1. Field of the Invention
The present invention relates to a technique for MR angiography and quantitative blood flow measurement, and more particularly, to techniques for MR angiography using steadystate transport-induced adiabatic fast passage and for quantitative MR flow measurement using pulsed adiabatic inversion.
2. Description of the Prior Art
The measurement of tissue perfusion, i.e., the flow of fluid in tissue, and of quantitative blood flow in arteries is important for the functional assessment of organs in vivo. Although the terms perfusion and flow are sometimes used interchangeably, perfusion as used herein is a quantifiable measurement of capillary blood flow which is generally measured indirectly in humans, while flow refers to the quantitative volume of blood passing through an artery in a given period of time. Angiograms, on the other hand, provide a qualitative view of arteries, tissues and the like which is useful for diagnostic purposes. Numerous techniques have been developed in the prior art for providing angiograms as well as measurements of perfusion and blood flow using magnetic resonance imaging.
For example, in U.S. Pat. application Ser. No. 746,771, filed Aug. 16, 1991, and in an article entitled "Magnetic Resonance Imaging of Perfusion Using Spin Inversion of Arterial Water", Proc. Natl. Acad. Sci., Vol. 89, pp. 212--216 (1992), one of the present inventors disclosed a method for measuring perfusion by labeling proton spins of inflowing water in the arterial blood using magnetic resonance. Continuous saturation or continuous inversion using an adiabatic excitation was then performed proximal to the tissue or organ of interest. In particular, perfusion was measured by labeling atoms in the blood at a base point, generating a steady state in the tissue or organ by continuing to label atoms until the effect caused by labeled atoms perfusing into the tissue or organ reaches a steady state, generating imaging information for the tissue or organ, and processing the imaging information to determine perfusion. In a particular embodiment of that invention, by continuously applying a radio-frequency (RF) field, spins associated with the atoms were inverted continuously by adiabatic fast passage. This technique was disclosed for determining perfusion in the brain as well as other tissues or organs having well defined arterial supplies such as the kidney, the liver and the heart.
Spin inversion was preferred for perturbing the magnetization of the arterial water in accordance with the technique of the above-identified application because it maximized the observed effect. In order to invert the arterial spins continuously as described therein, an RF signal was applied in the presence of a magnetic field gradient in the direction of the flow so that the movement of the spins through the magnetic field gradient leads to a change in the magnetic field through resonance. As described therein, this approach has lead to greatly improved images of tissue perfusion in the brain and other organs.
However, since the contribution from the arterial intravascular volume must be eliminated in order to get accurate measurements of tissue perfusion, the technique described in the aforementioned patent application is not suitable for quantitatively measuring blood flow or for acquiring an MR angiogram. On the contrary, the technique described in the aforementioned patent application requires symmetrical spoiler gradient pulses to be used in the imaging sequence around the 180.degree. pulse to eliminate arterial intravascular signals in both the proximal and the control saturation images. This minimizes the affects on intravascular spins of proximal saturation, which otherwise causes the difference image to include a contribution from the arterial intravascular volume in addition to a contribution caused by the exchange of labeled vascular water with tissue water during the perfusion measurement. In other words, spoiler gradients have been used when obtaining images of tissue perfusion in order to eliminate signal contribution from the moving spins. For this reason, while providing an excellent technique for measuring perfusion, the technique described in the aforementioned patent application cannot be used for MR angiography and quantitative blood flow measurement.
Recently, several subtractive time-of-flight (TOF) magnetic resonance angiography (MRA) techniques have been described in the art. For example, such techniques are described by Dixon et al. in an article entitled "Projection Angiograms of Blood Labeled by Adiabatic Fast Passage", Magnetic Resonance in Medicine, Vol. 3, pp. 454-462 (1986); by Nishimura et al. in articles entitled "MR Angiography By Selective Inversion Recovery", Magnetic Resonance in Medicine, Vol. 4, pp. 193-202 (1987), "Considerations of Magnetic Resonance Angiography By Selective Inversion Recovery", Magnetic Resonance in Medicine, Vol. 7, pp. 472-484 (1988), and "Magnetic Resonance Angiography By Selective Inversion Recovery Using A Compact Gradient Echo Sequence", Magnetic Resonance in Medicine, Vol. 8, pp 96-103 (1988); by Sardashti et al. in an article entitled "Spin-Labeling Angiography of the Carotids By Presaturation and Simplified Adiabatic Inversion", Magnetic Resonance in Medicine, Vol. 15, pp. 192-200 (1990); and by Wang et al. in an article entitled "Fast Angiography using Selective Inversion Recovery", Magnetic Resonance in Medicine, Vol. 23, pp. 109-121 (1992). The techniques described in those articles complement conventional phase-contrast techniques such as those described by Dumoulin et al. in an article entitled "Magnetic Resonance Angiography", Radiology, Vol. 161, pp. 717-720 (1986); by Wedeen et al. in an article entitled "Projective MRI Angiography and Quantitative Flow-Volume Densitometry", Magnetic Resonance in Medicine, Vol. 3, pp. 226-241 (1986); and by Nayler et al. in an article entitled "Blood Flow Imaging by Cine Magnetic Resonance", Journal of Computer Assisted Tomography, Vol. 10, pp. 715-722 (1986), and conventional time-of-flight MRA techniques such as those described by Gullberg et al. in an article entitled "MR Vascular Imaging With a Fast Gradient Refocusing Pulse Sequence and Reformatted Images From Transaxial Sections", Radiology, Vol. 165, pp. 241-246 (1987).
Dixon et al. describe an approach to MR angiography in which a constant RF pulse is transmitted through a separate surface coil in the presence of a constant magnetic gradient so that moving spins undergo adiabatic fast passage inversion. By turning the inversion RF on and off during alternate gated acquisitions, Dixon et al. generated images with and without labeled blood. An angiogram was then formed by simple subtraction for visualizing, for example, the carotid bifurcation.
In accordance with the Dixon et al. technique, all the blood passing the RF surface coil while the RF signal is being applied is labeled and then allowed to flow for a period of time so that it can enter the imaging volume. The longest possible labeling time is used which is consistent with the number of cardiac cycles chosen for TR. It is thus important in using the Dixon et al. technique that the RF signal be turned on and off slowly to prevent saturation of stationary tissue near the RF surface coil. It is also important that little power be used so as to prevent heating of the tissue of the patient in the vicinity of the RF surface coil. Moreover, in accordance with the technique of Dixon et al., since the phase encodings are interleaved and the labeled blood must be given time to propagate into the imaging volume before acquiring the image, only one phase encoding per cardiac cycle is possible. Furthermore, since this process is synchronized to the cardiac cycle by cardiac gating so as to phase encode every cardiac cycle with 90.degree. and 180.degree. pulses, the appropriate propagation time delay must also be selected to allow the labeled column of blood enough time to fill the arteries that are being imaged. Accordingly, Dixon et al. must estimate how far the spins travel in a selected period of time in order to maintain synchronization. An improved approach is desired which does not require synchronization to the cardiac cycle and which allows for fast scan imaging.
Another approach to MRA, referred to as selective inversion recovery (SIR) (as described in the aforementioned articles to Nishimura et al., Sardashti et al. and Wang et al.), uses spatially selective inversion pulses to label a column of arterial blood which then flows into an organ in the imaging region. Selective 180.degree. excitation inverts different regions between measurements to isolate arterial and/or venous blood so that high-resolution carotid artery angiograms and the like may be obtained. A 90.degree. presaturation pulse may also be applied to suppress the background intensity from static tissue in the angiogram, which is formed by subtracting an image acquired with inversion from an image acquired without inversion. As an inversion technique, SIR of the arterial inflow to an organ has been found to offer excellent angiographic contrast and penetration due to the fact that blood is inverted, not just saturated. As a subtractive technique, SIR also provides excellent background suppression, similar to that of phase contrast techniques.
However, there are also some problems associated with SIR techniques which use spatially selective inversion pulses to label blood flow. In general, these techniques must be gated to the cardiac cycle so that complete labeling of the inflowing blood occurs. In addition, the inversion pulses used to label blood flow should have precisely defined passbands in order for complete inversion to occur. This typically involves the use of complicated RF pulse shapes. Another drawback of SIR is the fact that spins in the more distal parts of the labeled column of blood undergo T1-relaxation as they traverse the slab thickness. The spins therefore lose part of the label before entering the imaging volume, leading to a loss of angiographic contrast. An improved angiographic technique is desired which overcomes such problems.
Various time-of-flight (TOF) methods have also been described in the prior art for quantitative NMR flow measurement. For example, Saloner et al. disclose in an article entitled "Flow Velocity Quantitation Using Inversion Tagging", Magnetic Resonance in Medicine, Vol. 16, pp. 269-279 (1990) and Edelman et al. disclose in an article entitled "Quantification of Blood Flow With Dynamic MR Imaging and Presaturation Bolus Tracking", Radiology, Vol. 171, pp. 551-556 (1989) methods employing spatially localized saturation or inversion RF pulses for labeling moving spins. In these approaches, a column of fluid is subjected to a series of spatially-selective RF pulses in a steady-state imaging experiment. This results in a banding pattern in the fluid from which the velocity may be determined. However, these methods are subject to errors arising from motion of the spins during application of the spatially selective RF pulses. In addition, the accuracy of these methods also depends on the characteristics of the spatially-selective inversion pulses used to tag the fluid. In general, optimized RF pulses must be used in order for accurate velocity measurements to be made. Unfortunately, due to the fact that each group of "tagged" spins experiences multiple RF pulses during transit through the inverted slab, contrast between bands decreases as the flow moves downstream. This effect makes it more difficult to observe the evolution of the banding pattern as the fluid flows distal to the inversion plane.
Lee et al. describe another flow measurement technique in an article entitled "Spatially Resolved Flow Velocity Measurements and Projection Angiography by Adiabatic Passage", Magnetic Resonance Imaging, Vol. 9, pp. 115-127 (1991), which provides direct assessment of in-plane and oblique directional flow velocities and visualization of flow velocity profiles as well as flow angiography based on the time-of-flight technique. In particular, Lee et al. generate a band in the liquid which may be used for measuring flow velocity by applying a fast adiabatic passage pulse after a cardiac gated sequence and then applying a spin echo sequence for acquiring the image. A 180.degree. RF pulse is also applied for stationary spin suppression prior to the adiabatic fast passage pulse and a 90.degree. RF pulse is applied prior to phase encoding. Unfortunately, Lee et al. do not sequence fast enough to see plural bands in a single cardiac cycle, and since the 90.degree. and 180.degree. gradient pulses are used, fast scanning techniques cannot be used. Hence, the system of Lee et al. does not provide optimized time resolution of the images taken during the cardiac cycle. An improved MR flow measurement technique is desired which will allow for fast scanning of the slab so as to improve time resolution.
The present invention has been designed to overcome the aforementioned limitations in the prior art by providing a comprehensive system for MR angiography and MR blood flow measurement for different regions of the body. As will be clear from the following description, the present inventors have found that improved MR images of arteries, veins and tissues may be produced using a steady-state adiabatic inversion technique and that blood flow may be measured by adiabatically inverting spins in the direction of the magnetic gradient. The present invention is thus believed to meet long felt needs in the prior art in that it provides for the first time a methodology which allows for MR angiography as well as quantitative blood flow and tissue perfusion measurement of any tissue or organ having a well defined arterial supply.